Update CHANGES.

CHANGES

+Galène 0.3 (unreleased):
+
+  * Implemented the command /muteall.  This allows muting all users except
+    yourself.
+  * Implemented testing of TURN servers, on both the client and the server
+    side.  Connectivity will be checked by the server every 12 hours, and
+    by the client whenever a user uses the /relay-test command.
+  * Group configurations are now checked at startup, and any failures are
+    logged.  This implies that it is no longer possible to use undefined
+    keys in group definition files (this is a breaking change).
+  * Subgroups can now be public.
+  * Implemented client-side filters that are run over the video before it
+    is sent to the server.  A pair of mirroring filters are provided,
+    which are useful when using a mirror in front of a webcam.
+
 9 January 2021: Galène 0.2
 
   * We now work around a limitation of the Go runtime that used to cause